Side-entry specimen positioning systems find extensive application in electron microscopy. Such specimen positioning systems are described in U.S. Pat. Nos. 3,745,341 and 3,778,621. FIG. 1(a) is a cross-sectional view of a conventional side-entry specimen positioning device. FIG. 1(b) is a cross-sectional view taken on line A--A' of FIG. 1(a). In these figures, an objective lens comprises a yoke 1, an excitation coil 2, an upper magnetic pole piece 3, and a lower magnetic pole piece. 4. The gap between the pole pieces 3 and 4 is maintained by a spacer 5. A stage 6 is installed in the yoke 1. A rotatable cylinder 7 is mounted to the stage 6 via a spacer 8 and a bearing 9. A specimen holder 11 extends through the cylinder 7. A specimen S is held to the front end of the holder 11 via a ball joint 10. A Z-axis adjusting screw 12 and a Y-axis adjusting screw 13 are mounted to the cylinder 7 to push that ball joint 10 in the Z and Y axes, respectively. Springs S.sub.1 and S.sub.2 are mounted between the cylinder 7 and the joint 10 and disposed opposite to the screws 12 and 13, respectively. The specimen holder 11 is capable of sliding inside the ball joint 10. The holder 11 is inserted between the pole pieces 3 and 4 from the X direction so as to cross the optical axis Z of the electron beam within the X-Y plane perpendicular to the optical axis Z. The specimen S can be rotated in the Z and Y directions about the ball joint 10 by rotating the Z-axis adjusting screw 12 and the Y-axis adjusting screw 13, respectively. Since the rotatable cylinder 7 is able to turn about the X axis perpendicular to the optical axis Z, the specimen S can be tilted to the optical axis Z by rotating the cylinder 7 in the direction indicated by the arrow C. The front end of the specimen holder 11 is retained by a pivot pin 14 and a pivot bearing 15. This bearing 15 is slidably fitted in a retainer 16 mounted to one side of the yoke 1. The bearing 15 is held by an X-axis adjusting screw 17 mounted to the retainer 16. The pivot pin 14 is supported by a spring (not shown) which is mounted to the pivot bearing 15. The specimen holder 11 can be moved in the X direction by rotating the X-axis adjusting screw 17.
It is well known that the resolution of an electron microscope is enhanced as the focal length of the objective lens is reduced. It is also well known that as the space between the upper pole piece 3 and the lower pole piece 4 is reduced, the focal length of the objective lens decreases, provided that the diameters of the pole pieces 3 and 4 of the objective lens are maintained constant. In the aforementioned specimen positioning device of the electron microscope, the front end of the specimen holder 11 is retained by the pivot pin 14 and the pivot bearing 15 and, therefore, the bearing 15 and the pin 14 are pressed against the front end cf the holder 11 when the X-axis adjusting screw 17 is rotated. Further, the atmospheric pressure acts on the specimen holder 11, because the inside of the yoke 1 is a vacuum. For this reasons, the front end of the holder is pressed against the pivot pin 14 with a greater force. In addition, a moment due to the weight of the pin 14 itself acts on the front end of the holder. In this way, a plurality of forces act on the front end of the specimen holder 11. Therefore, if the portion of the holder 11 which holds the specimen is thinned, then the specimen S bends. Accordingly, the front end of the specimen holder which grips the specimen must be thickened. Consequently, the space between the pole pieces 3 and 4 of the objective lens must be increased.
In recent years, a specimen positioning device which is free of the foregoing problems has been proposed. FIG. 2(a) is a cross-sectional view of this device. FIG. 2(b) is a cross-sectional view taken on line B--B' of FIG. 2(a). In these figures, an objective lens comprises a yoke 21, an excitation coil 22, an upper magnetic pole piece 23, and a lower magnetic pole piece 24. The gap between the pole pieces 23 and 24 are maintained by a spacer 25. A stage 26 is installed in the yoke 21. A rotatable cylinder 27 is mounted to the stage 26 via a spacer 28 and a bearing 29. A specimen holder 31 extends through the cylinder 27. A specimen S is held to the front end of the holder 31 via a ball joint 30. A Z-axis adjusting screw 32 and Y-axis adjusting screw 33 are mounted to the cylinder 27 to push the joint 10 in the Z and Y directions, respectively. Springs S.sub.1 and S.sub.2 are mounted between the cylinder 27 and the ball joint 30 and located opposite to the adjusting screws 32 and 33, respectively. A tapering insulator 34 is fixed to the outer periphery of the specimen holder 31 and protrudes from the front end of the cylinder 27 toward the optical axis Z of the electron beam. A spherical member 35 is fixed to the outer periphery of the insulator 34. A support 36 is fitted in the side wall of the yoke 21 perpendicularly to the X axis. An arm 37 is mounted inside the support 36 in such a way that the arm can be rotated about a pin 38 by rotating an X-axis adjusting screw 39. The front end cf the arm 37 is provided with a round hole through which the specimen holder 31 extends. Two bushings 40 and 41 are mounted around the hole. One of the bushings is located above the plane defined by the X axis and the optical axis, while the other is located below the plane. The central axis of the bushings 40 and 41 agrees with the central axis of the spherical member 35. The bushings 40 and 41 are fixed to the arm 37 such that they are in contact with the spherical member 35. A pivot bearing 42 is mounted between the arm 37 and the X-axis adjusting screw 39. A spring 43 is mounted between the arm 37 and the support 36.
In this specimen positioning device, the inside of the yoke 1 is a vacuum and so the atmospheric pressure acts on the specimen holder 31. This pressure is transmitted to the bushings 40 and 41 via the spherical member 35. When the arm 37 is rotated by rotating the x-axis adjusting screw 39, the bushings 40 and 41 are rotated about the pin 38. At this time, the spherical member 35 slides on the bushings 40 and 41. Then, the specimen holder 31, or the specimen S, is moved in the X direction by the sliding movement of the spherical member 35. The holder 31 can be rotated about the ball joint 31 in the Z and Y directions by rotating the Z- and Y-axis adjusting screws 32 and 33, respectively. Since the atmospheric pressure causes the spherical member 35 to press the specimen holder 31 against the bushings 40 and 41 and all times, the holder 31 rotates while sliding in the ball joint 30. Thus, the spherical member 35 rotates while sliding on the bushings. The specimen S moves in the Y or Z direction. Since the rotatable cylinder 27 can rotate about the X axis perpendicular to the optical axis Z of the electron beam, when the cylinder 27 is rotated in the direction indicated by the arrow C, the holder 31 turns about the X axis. Simultaneously, the spherical member 35 slides on the bushings 40 and 41. Consequently, the specimen S can be inclined to the optical axis Z.
In this manner, in the specimen positioning device shown in FIGS. 2(a) and 2(b), neither the atmospheric pressure nor the moment arising from the weight of the pivot pin itself is applied to the front end of the specimen holder 31 which grips the specimen and, therefore, the front end can be made thin. This enables the gap between the pole pieces 3 and 4 to be narrowed. Hence, the resolution of the electron microscope can be enhanced.
In this specimen positioning device of the electron microscope, after the specimen holder holding plural specimens is placed in position within the objective lens, the inside of the lens is evacuated to a high vacuum. Then, the specimens held by the holder are successively observed. If new specimens are to be observed, the inside of the objective lens is once returned to the atmospheric pressure. Subsequently, the new specimen holder is set in the lens. The inside of the lens is evacuated to a high vacuum. Thereafter, the specimens held by the holder are observed one after another. In this way, specimens which can be observed after the inside has been once evacuated are few in number, and it takes a long time to evacuate the inside to a high vacuum. Consequently, it is impossible to observe a number of specimens in a short time.
Where specimens are heated, cooled, subjected to an evaporation process, or otherwise processed before placed in position within the objective lens, a chamber is formed for the processing. However, in the specimen positioning device shown in FIGS. 2(a) and 2(b), the chamber for the processing must be located inside the specimen holder 31 which is opposite to the optical axis. Therefore, the holder 31 is long and susceptible to external vibration. This also hinders the observation of specimens.